How Digital Cameras Capture Images 2

Shutter
The shutter is a complicated mechanism that precisely controls the duration of time
that light passing through the lens remains in contact with the digital image sensor.
The camera’s shutter is activated by the shutter release button.
Prior to the digital age, the shutter remained closed to prevent the film from being
exposed. Depending on the type of digital image sensor, a mechanical shutter may not
be necessary. Rather than a shutter revealing light to initiate a chemical reaction in the
film, the digital image sensor may simply be turned on and off.

Shutter Speed
Shutter speed refers to the amount of time the shutter is open or the digital image
sensor is activated. The exposure of the image is determined by the combination of
shutter speed and the opening of the aperture. Shutter speeds are displayed as
fractions of a second, such as 1/8 or 1/250. Shutter speed increments are similar to
aperture settings, as each incremental setting either halves or doubles the time of the
previous one. For example, 1/60 of a second is half as much exposure time as 1/30 of a
second, but about twice as much as 1/125 of a second.
Photographers often use shutter speeds to convey or freeze motion. A fast-moving
object, such as a car, tends to blur when shot with a slow shutter speed like 1/8. On the
other hand, a fast shutter speed, such as 1/1000, appears to freeze the blades of a
helicopter while it’s flying.

Using Reciprocity to Compose Your Image
You can adjust the aperture setting and shutter speed to create several different
correctly exposed images. The relationship between the aperture and shutter is known
as reciprocity. Reciprocity gives the photographer control over the depth of field of the
image, which controls the area of the image that remains in focus. This is the easiest
way to control what part of the image you want the viewer to pay attention to.
For example, opening the lens aperture by one stop and decreasing the shutter
speed by one stop results in the same exposure. Closing the aperture by one stop
and increasing the shutter speed by one stop achieves the same exposure as well.
Therefore, f4 at 1/90 of a second is equal to f5.6 at 1/45 of a second. The reason is that
the camera’s aperture setting and shutter speed combine to create the correct
exposure of an image.

Digital Image Sensor
When the reflective light from the photographed subject passes through the lens and
aperture, the image is captured by the digital image sensor. A digital image sensor is the
computer chip inside the camera that consists of millions of individual elements capable
of capturing light. The light-sensitive elements transform light energy to voltage values
based on the intensity of the light. The voltage values are then converted to digital data
by an analog-to-digital converter (ADC) chip. This process is referred to as analog-todigital
conversion. The digital numbers corresponding to the voltage values for each
element combine to create the tonal and color values of the image.

Each light-sensitive element on a digital image sensor is fitted with either a red, green,
or blue filter, corresponding to a color channel in a pixel in the image that is captured.
There are roughly twice as many green filters as blue and red to accommodate how the
eye perceives color. This color arrangement is also known as the Bayer pattern color filter
array. (For more information on how the eye perceives color, see “Understanding How
the Eye Sees Light and Color” on page 29.) A process known as color interpolation is
employed to ascertain the additional color values for each element

Common Types of Digital Image Sensors
There are two types of digital image sensors typically used: a charge-coupled device
(CCD) and a complementary metal oxide semiconductor (CMOS).
CCD
CCD sensors were originally developed for video cameras. CCD sensors record the
image pixel by pixel and row by row. The voltage information from each element in the
row is passed on prior to descending to the next row. Only one row is active at a time.
The CCD does not convert the voltage information into digital data itself. Additional
circuitry is added to the camera to digitize the voltage information prior to transferring
the data to the storage device.

Megapixels
A camera’s resolution capability is measured in megapixels. This measurement is based
on the number of millions of pixels of image information that can be captured by the
light-sensitive elements on the digital image sensor. Thus, a 15 megapixel camera is
capable of capturing 15 million pixels of information.
ISO
Traditionally, the International Standards Organization (ISO) has provided a benchmark
rating of the relative sensitivity of film. The higher the ISO rating, the more lightsensitive
a particular film is. Higher ISO films require less light to record an image. The
ISO rating has been redefined for digital cameras, indicating the image sensor’s
sensitivity to light. Most DSLRs have ISO settings from 100 to 3200 ISO.
Unfortunately, at higher ISO settings (400 ISO and above), some cameras have difficulty
maintaining consistent exposure for every single pixel in the image. To increase the
sensitivity of the digital image sensor in these situations, the camera amplifies the
voltage received from each image sensor element prior to converting the signal to a
digital value. As the voltage signals from each element are amplified, so are anomalies
within solid dark colors. This results in sporadic pixels with incorrect bright color values,
also known as digital noise. For more information on digital noise, see “Reducing Digital
Noise” on page 25.

Memory Card
After the digital image sensor has captured the image, the camera employs a series of
processes to optimize the image. Many of these processes are based on camera
settings established by the photographer prior to taking the shot, such as the ISO
setting. After image processing, the camera stores the digital information in a file. The
type of digital file created varies depending on the camera’s manufacturer. However,
the camera’s RAW file contains the digital image data before it has been converted to a
standardized file type, such as JPEG or TIFF. Not all RAW files are alike, but the image
data produced by your camera’s digital image sensor and processor is retained bit for
bit in that file. For more information about these file types, see “Understanding RAW,
JPEG, and TIFF” on page 21.
Once the file is ready for storage, the camera transfers the file from its processor to the
memory card. There are several types of memory cards, but the process by which they
receive the information is the same.
External Flash
Certain photographic situations require the additional light provided by an external
flash. Many prosumer DSLR models have built-in or on-camera flashes, but the
proximity to the lens and the lack of flash exposure control prevent their use in
professional situations.
External flashes provide professional-level control over flash exposure. This allows for
fine-tuned fill flash (low-intensity flash that illuminates the subject against a bright
background so the subject does not appear in silhouette) and the prevention of
overexposed subjects in close-quarter situations.
External or off-camera flashes are synced to the shutter release via the hot-shoe
bracket or PC terminal.
Hot-shoe bracket
PC Terminal

Understanding RAW, JPEG, and TIFF
It’s important to understand the differences between image file types. RAW, JPEG, and
TIFF file types are described below.
RAW
A camera’s RAW file is an uninterpreted, bit-for-bit digital image recorded by the
camera when the image is captured. Along with the pixels in the image, the RAW file
also contains data about how the image was shot, such as the time of day, the
exposure settings, and the camera and lens type. This information is also known as
metadata
. RAW refers to the state of the image file before it has been converted to a
common format, such as JPEG or TIFF. Because most photography applications
previously could not process RAW files, RAW files had to be converted before they
could be used in image processing software.

Why Shoot RAW Files?
There are many reasons to capture images as RAW files rather than JPEG files.
However, it’s important to note that RAW image files require additional work to
achieve the color balance you’re looking for, whereas JPEG files are color-balanced by
the camera for you. JPEG files are also smaller than RAW image files, requiring less
storage space.
The advantages to shooting RAW files are:
Increased bit depth allows for more color-correction “head room.” The JPEG format is
limited to 8 bits per color channel. RAW images store 16 bits per channel, with
12 to 14 bits per channel of color information. Although it may sound confusing, this
means you can do significantly more color correction without degrading the image
or introducing color noise. (For more information about bit depth, see “Learning
About Bit Depth” on page 38.)
After the RAW file is decoded, you work with the most accurate and basic data
about an image.
You control the white balance, color interpolation, and gamma correction aspects
of the image during post-production rather than when shooting.
The image file isn’t compressed, as JPEG files are, which means that no image data
is lost.
Most cameras are capable of and do shoot color outside the gamut range of JPEG
(both Adobe RGB 1998 and sRGB), which means color clipping occurs when you
shoot JPEG files. RAW files preserve the camera’s original image gamut, allowing
Aperture to make image adjustments that take advantage of the full range of
captured colors.
RAW files give you control of noise reduction (luminance and color separation) and
sharpening after capture. JPEG noise reduction and sharpening are permanently
applied to the image according to the settings on the camera.

JPEG
JPEG (Joint Photographic Experts Group) is a popular image file format that lets you
create highly compressed image files. The amount of compression used can be varied.
Less compression results in a higher-quality image. When you shoot JPEG images, your
camera converts the RAW image file into an 8-bit JPEG file (with 8 bits per color
channel) prior to saving it to the memory card. In order to accomplish this, the camera
has to compress the image, losing image data in the process. JPEG images are
commonly used for online viewing.
TIFF
TIFF (Tag Image File Format) is a widely used bitmapped graphics file format capable of
storing 8 or 16 bits per color channel. Like JPEG files, TIFF files are converted from RAW
files. If your camera does not have an option to shoot TIFF files, you can shoot RAW files
and then convert them to TIFF files using software. TIFF files can have greater bit
depths than JPEG files, allowing them to retain more color information. In addition, TIFF
files can use lossless compression, meaning that although the file gets a little smaller,
no information is lost. The end result is greater image quality. For these reasons,
printing is commonly done from TIFF files.

How Digital Cameras Capture Images 3

How Digital Cameras Capture Images 2

How Digital Cameras Capture Images 1

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